1. Field of Invention
The present invention relates to a trommel screen, especially to a trommel screen system for separating fly ash from filter granules in the hot gas granular moving bed filter with a dynamically real-time adaptive function.
2. Description of Related Art
Granular moving bed filters have been developed for high-temperature flue gas cleanup. In principle, the filter granules are chemically inert, although they have been considered to remove particulates, e.g. fly ash and alkali from PFBC and IGCC flue gases simultaneously.
The concentration of fly ash in the flue gas (syngas), entering granular moving bed apparatus (e.g. granular moving bed filter or moving bed adsorber) changes in time, according to type of fuel, character of gasification or conditions of burning. To keep the filtration efficiency in granular moving bed filter/adsorber stable and uniform in time, it is necessary to optimize the mass flow rate of granular filter medium in moving bed. When the concentration of fly ash in flue gas is high, it is advisable to increase the velocity of filter granules and remove dirty filter granules with fly ash from moving bed apparatus faster. When the concentration of fly ash is low, it is suitable to decrease the mass flow rate of granules in moving bed. The mass flow rate of granular filter/sorbent medium is controlled by a rotary feeder under the feeding hopper of trommel screen system.
Granular filter/sorbent medium in moving bed can flow continuously or intermittently. The advantage of intermittent (batch mode) running of moving bed consists in formation of thin filter cake on the contact surface of moving bed with flue gas. This thin filter cake increases efficiency of filtration while running moving bed at relatively low pressure drop of gas.
Dirty granular filter/sorbent medium (e.g. filter granules with fly ash or spent sorbent granules with rest of fly ash) from moving bed apparatus is introduced into the trommel screen, where filter granules or sorbent granules and fly ash are separated. To keep a high level in separation efficiency, it is useful to run this equipment continuously, even when moving bed granules enter the trommel screen with low or high fly ash concentration and in continuous or intermittent (batch mode) cycle.
Separation efficiency of fly ash from moving bed granules depends predominantly on the residence time of mixture of moving bed granules and fly ash in the trommel screen. A reduction of rotating speed influences the separation efficiency negatively. On the contrary, the inclination of trommel screen influences the residence time of moving bed granules significantly. So, it is advisable to change the inclination of central axis of the trommel screen according to the mass flow rate of fly ash leaving the revolving trommel screen. The mass flow rate of fly ash could be measured by an impact flow meter.
For example, in the prior art, German patent No. 3546133 describes apparatus and method to the washing and filtration (dewatering) a slurried mineral mixture in a rotary, vibrated and perforated drum. Oscillations of the drum are controlled by hydraulic cylinders. Longitudinal axis of the drum is adjusted inclined in the process of filtration. Its inclination can be changed according to flowability of slurry in order to affect continuous flow rate and residence time of slurried mineral mixture in rotary drum. According to UK patent No. 2438076, a rotary screen drum has an inlet at a lower level than an outlet, such that if the apparatus becomes blocked, material flows of preference back into the inlet rather than out of the outlet. In other terms, the screen drum is angled upwards from inlet to outlet. During normal usage, when not blocked, the drum may be used in a horizontal orientation. The drum is driven rotationally by a motor, and an internal screw thread arrangement may progress material along the length of the drum. The orientation of the drum may be adjusted with an actuator.
Referring to German Utility Patent No. 20022079 U1, the central axis of the trommel screen is horizontal or inclined. The transport of bulk material in trommel screen is influenced by the inclination angle of trommel screen and by guidance sheet metals inside it. Furthermore, in the German Utility Patent No. 8808236 U1, the base frame of trommel screen has a turn-around axle in order to be able to adjust the inclination of the central axis of the trommel screen. Additionally, in the German Utility Patent No. 20314575 U1, the sieve assembly comprises a driven trommel screen with a horizontal central axis, of which the inclination is adjustable.
All above mentioned apparatuses are set to fixed inclination of cylindrical or polygonal drum. The angle of inclination can be changed providing that trommel screen is put out of operation. Fast reaction to changing operation conditions such as a change of dust concentration in flue gas or change of mass flow rate of filter granules in moving bed is not possible.
Although the above prior art can screen out fresh sand whose filter granule size falls in operating diameter range (0.1 mm-10 mm), in practice the apparatus is still unable to meet general demands.
Thus there is a need to provide a dynamically real-time adaptive trommel screen that dynamically adjusts operation of the trommel screen in time according to the mass flow rate of the screened residues so as to improve the screening (separation) efficiency and match users' requirements.